In the past, most machine parts of automobiles, industrial machinery, etc. have generally been made by hot forging steel rods made of medium carbon steel or low carbon steel into the part shapes, then reheating them and heat treating them by quenching and tempering to thereby impart high strength and high toughness.
However, this heat treatment requires tremendous heat energy. Further, the treatment steps increase and the semifinished parts increase etc., so the ratio of heat treatment costs in the costs of production of parts becomes greater. For this reason, to simplify the production process and lower the heat treatment costs when producing such structural parts, hot forging use non heat-treated steel omitting the heat treatment of quenching and tempering has been developed.
Hot forged parts made using non heat-treated steel have been heated once to 1200° C. or more and forged at a high temperature of 1000 to 1200° C. or so. However, heating at 1200° C. or more causes the austenite grains to become coarser. By forging at a high temperature of 1000 to 1200° C., recrystallization progresses after working, the ferrite-pearlite obtained in the cooling process becomes coarser, and therefore hot forging non heat-treated parts using non heat-treated steel generally become smaller in yield strength ratio and impact value compared with heat-treated steel parts.
To solve these problems, Japanese Patent Publication (A) No. 55-82749 describes to increase the amount of Mn of the steel for use in machine structures and further add a small amount of V, Japanese Patent Publication (A) No. 55-82750 describes to add a small amount of V to steel for use in machine structures, and further Japanese Patent Publication (A) No. 56-169723 describes to control the ingredients and also cool in the cooling process after forging by a rate of 0.7° C./sec or less in the temperature range of 1000 to 550° C. so as to make a large amount of intergranular ferrite of cores of MnS disperse in the steel and as a result obtain a fine-grain structure and improve the toughness and fatigue characteristics. However, the ferrite-pearlite obtained by these methods remains coarse and therefore the amount of increase in impact value or strength due to increasing the fineness of the structure is small at the present.
Recently, for protection of the global environment, better fuel economy of automobiles has been increasingly demanded. One of the effective means for achieving better fuel economy of automobiles is reduction of the weight of the vehicles. This is leading to reduction of the size of parts by improvement of part strength. However, the current ferrite-pearlite type non heat-treated steel has a limit of strength of about 1000 MPa. It is becoming impossible to meet the recent demands for higher strength and higher toughness.
On the other hand, to obtain both a 1000 MPa or more strength and a high toughness, it is necessary to make the structure a martensite structure or bainite structure in which carbides are finely dispersed.
Numerous art regarding non heat-treated steel given a martensite or bainite structure as hot forged has been proposed up to now. For example, Japanese Patent Publication (A) No. 1-129953 describes that by making the amount of carbon a relatively low one of 0.04 to 0.20% so as to raise the Ms point aiming at the effect of self tempering and, further, adding Ti, B, and other elements to increase the quenchability and further rapidly cooling after forging to make the structure a martensite or bainite structure or a mixed structure of martensite and bainite, a high strength and a good toughness are obtained. Further, Japanese Patent Publication (A) No. 63-130749 describes increasing the N without adding Ti and B and rapidly cooling from the Ar3 point or more.
However, with the high strength disclosed in these Japanese Patent Publication (A) No. 1-129953 and Japanese Patent Publication (A) No. 63-130749, there is little effect of improvement of the machineability even if adding Ca, Te, Bi, or other machineability improving elements.
Further, Japanese Patent Publication (A) No. 2000-129393 discloses the discovery that by adding suitable amounts of Mn and Cu together, a high yield strength and good toughness are obtained and that by adding suitable amounts of Ti and Zr and making Ti carbosulfides and Zr carbosulfides finely disperse, the amount of formation of MnS is reduced and in turn the machineability of the steel material is improved. However, Ti carbosulfides and Zr carbosulfides are hard, so sometimes cause tool damage and promote tool wear at the time of machining. Whatever the case, it is not easy to obtain steel and machine parts with high strength and high toughness and with superior machineability.